1. Field of the Invention
The present invention relates generally to a Lundell type rotor of an alternator mounted on a vehicle, and more particularly to the rotor wherein a plurality of claw poles disposed along a circumferential direction of the rotor are joined to a claw pole joint ring to be fixed to one another.
2. Description of Related Art
An alternator used for a vehicle has a cylindrical stator and a columnar rotor disposed in a center space of the stator and generates an alternating current from a rotational force applied to the rotor. The rotor such as a Lundell type rotor has a rotational shaft, a rotor core rotated with the shaft, and a field coil wound on the core. The rotor core is composed of two Lundell type pole cores disposed on front and rear sides of the alternator. Each pole core has a boss portion fitted to the shaft and surrounded by the field coil, a disk portion, and a plurality of claw poles. The disk portion extends from an end of the boss portion opposite to the other pole core along a radial direction of the rotor. Each claw pole extends from a radial directional end of the disk portion along an axial direction of the rotor so as to surround the field coil with the claw poles. The claw poles of the front side pole core and the claw poles of the rear side pole core are alternately arranged along the circumferential direction. The alternator with the Lundell type rotor generally has a brush apparatus to feed a field current to the coil rotated with the rotor core. This rotor is called a brush Lundell type rotor.
Further, as another type of rotor, a Lundell type rotor with a stationary field coil has been disclosed in Published Japanese Patent First Publication No. H06-261503. In this rotor, claw poles of a rear pole core are fixed to claw poles of a front pole core, a stationary iron core portion is fixed to a rear frame of an alternator, and a stationary filed coil is wound around the stationary iron core portion. The stationary iron core portion and the field coil are disposed on the inner side of the pole cores in the radial direction. Although the pole cores with the claw poles are rotated with a rotational shaft in response to a rotational force, the filed coil remains stationary. Therefore, there is no brush apparatus in this alternator. A field magnetic flux formed by the field coil passes through the claw poles of the front pole core, a stator core, the claw poles of the rear pole core, the stationary iron core portion, and a smaller diameter portion (also called a boss portion) of the front pole core in that order, and the flux is returned to the claw poles of the front pole core.
The claw poles of the rear pole core are fixed to the claw poles of the front pole core through a claw pole joint ring. This joint ring is made of a non-magnetic metal such as 18-8 austenite stainless steel (or SUS304 prescribed by Japan Industrial Standards (JIS)). Each claw pole has an inner side surface facing the field coil along the radial direction, and the joint ring is joined to the inner side surfaces of the claw poles of the front and rear pole cores by welding so as to be placed between the field coil and the pole cores in the radial direction.
This joint ring is also used for the brush Lundell type rotor to suppress vibrations of claw poles. The joint ring for the brush Lundell type rotor is called a damper ring. A brush Lundell type rotor with a damper ring has been disclosed in each of Published Japanese Patent First Publications No. H05-146126 and No. H11-136914. In these rotors, the damper ring is formed by bending a non-magnetic metallic band plate in a ring shape and connecting both ends of the band plate with each other by welding.
More specifically, the damper ring disclosed in the Publication No. H05-146126 is joined to proximal portions of the claw poles of the front and rear pole cores. Because the extending direction of the claw poles of the front pole core is opposite to that of the claw poles of the rear pole core, the damper ring is bent toward the axial direction every claw pole pitch. In the rotor disclosed in the Publication No. H11-136914, both side ends of the damper ring in the axial direction are bent toward the inner side of the rotor along the radial direction so as to be formed in a U shape in section.
Various requirements are imposed on the joint ring and damper ring to stably fix the claw poles to one another. Particularly, in case of the Lundell type rotor with a stationary field coil, the claw poles of the rear pole core are required to have high resistance to the centrifugal force induced by the rotation of the core. Therefore, it is required to produce the joint ring at high strength and with high reliability.
When the joint ring is produced by connecting both ends of a band plate with each other by welding or the like, the joint ring inevitably has a welded portion. This welded portion is required to have the same joint strength (or tensile strength) as that of a non-welded portion of the joint ring. When the strength of the welded portion is lower than that of the non-welded portion, the strength of the joint ring is restricted to that of the welded portion. Further, to obtain the welded portion having the same tensile strength as that of the non-welded portion, it is required to precisely or uniformly weld the whole surface of one end to the whole surface of the other end of the band plate. However, it is not easy to precisely or uniformly join the ends of the band plate to each other by welding or the like. Therefore, it is difficult to ensure that the welded portion has the same strength as that of the non-welded portion.
Further, the claw poles of the alternator are vibrated at the same frequency as that of a change in the strength of the magnetic field. Therefore, the joint ring is fatigued. To prevent the fatigue failure of the joint ring, it is required to produce the joint ring having a sufficient fatigue life.
Moreover, after the joint ring is formed in a ring shape by welding, the joint ring is again welded to the claw poles. Therefore, there is a high probability of heat deterioration occurring in the joint ring. More specifically, crystal conditions of the non-magnetic metal are changed due to the thermo-mechanical processing so as to locally change magnetic characteristics of the joint ring, so that the joint ring is locally weakened in strength or the non-magnetization of the joint ring is lost.
Furthermore, the centrifugal force and/or vibration applied on a portion of the joint ring attached to each claw pole differ from those applied on another attached portion of the joint ring. Therefore, torsion is applied on the joint ring. To prevent the joint ring from being distorted, there is an idea that a sectional shape of the joint ring is not uniformly set. That is, the sectional shape of the joint ring at an attached portion is differentiated from that at another attached portion while considering the centrifugal force and/or vibration applied on the joint ring. However, the manufacturing of the rotor is complicated, and a space for the field coil is narrowed.